Hollow turbomachine blade

ABSTRACT

A hollow turbomachine blade including an internal cooling passage, an open cavity situated at the free end of the blade and defined by an end wall and the side wall of at least one rim which extends between the leading edge and the trailing edge of the blade, and at least one cooling channel connecting said internal cooling passage to said open cavity, said cooling channel opening out at the base of the rim and the wall of the rim forming an angle relative to said end wall that is obtuse, being strictly greater than 90°. An indentation may be formed in the wall of the rim at the outlet from said cooling channel. Said blade advantageously does not include a pressure rim.

The invention relates to a hollow turbomachine blade, and moreparticularly it can relate to a hollow blade of the type for ahigh-pressure gas turbine rotor in a turbojet.

BACKGROUND OF THE INVENTION

As shown in FIGS. 1 and 2, it is known to provide an open cavity 5 or“bathtub” at the free end 3 of a hollow blade 2, the cavity beingdefined by an end wall 7 that extends over the entire end of the blade,and by side walls in the form of two rims 9 and 10 that extend betweenthe leading edge 12 and the trailing edge 14 of the blade, one of thetwo rims 9 extending the pressure side wall 8 of the blade while theother rim extends the suction side wall 11 of the blade. These rims arereferred to herein as the pressure rim and the suction rim.

The rubbing surfaces between the free end 3 of each blade and theannular surface of the turbine casing 16 that surrounds the blades, asshown in FIG. 2, are thus restricted to the rims 9 and 10 so as toprotect the body of the blade, and more particularly its end wall 7.Furthermore, the rims 9 and 10 serve to optimize the clearance J betweenthe free end of the blade 3 and the casing 16, and thus limit the amountof gas that can pass from the pressure side to the suction side, therebygenerating aerodynamic losses that are harmful to the efficiency of themachine.

Because of the high temperatures of the gas passing through the turbineand the high speeds of rotation of the blades, it is necessary to coolthe end rims 9 and 10 so as prevent them from deteriorating under thecombined effect of friction and heat. Thus, cooling channels aregenerally provided to connect the internal cooling passage 18 of thehollow blade to the open cavity 5 so as to deliver cool air to the rims9 and 10.

Furthermore, recent studies have shown that the absence of the pressurerim 9 makes it possible to overcome problems of this rim deterioratingwhile nevertheless still guaranteeing turbine efficiency that is as goodas or even better than the efficiency of a blade having two rims, one onthe pressure side and another on the suction side.

However, the absence of a pressure rim 9 makes it necessary to improvethe cooling of the suction rim 10, since it is no longer protected fromhot gas by the pressure rim 9.

In conventional blades having two rims as described in documents EP 0816 636 B1 and EP 1 270 873 A2, the channels dedicated to cooling thesuction rim cool it poorly, either because they are located too far awayfrom said rim, or because they open out in the vicinity of the top endof the rim. At best, they cool only the top end of the suction rim.

Document EP 1 422 382 A2 presents a solution for improving the coolingof the suction rim of a blade that does not have a pressure rim. Thatsolution consists in making notches in the side wall of the suction rimfacing the open cavity and impacted by the hot gases. Said notchesextend substantially from the base of the suction rim to the top end ofsaid rim, and cooling channels are pierced at the bottoms of thesenotches down to the internal cooling passage in the blade. The maindrawback of those notches are that they weaken the suction rim, inparticular at its top end, and they confine the cool air so that thewall portions that are situated between the notches are cooled not atall (or very poorly). In addition, depending on the shapes of thenotches, it can be difficult to drill the cooling channels, and thatoperation can require specific equipment.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to provide a hollow blade for aturbomachine that is simple to manufacture and in which the end rim(s)is/are well cooled.

To achieve this object, the invention provides a hollow turbomachineblade including an internal cooling passage, an open cavity situated atthe free end of the blade and defined by an end wall and the side wallof at least one rim extending between the leading edge and the trailingedge of the blade, said blade further including at least one coolingchannel connecting said internal cooling passage to said open cavity,and said cooling channel opens out in the vicinity of the junction zonebetween the end wall and the side wall of the rim, i.e. at the base ofthe rim, and the side wall of the rim forms an obtuse angle, strictlygreater than 90°, relative to said end wall.

The advantages of the invention are associated firstly with theinclination of the side wall of the rim relative to the direction thatis orthogonal to the end wall, which direction is considered below asbeing vertical, and secondly with the fact that the cooling channels areformed at the base of the rim.

This disposition allows the cooling air escaping from the channels torun along the wall of the rim and thus form a protective air film onsaid wall, protecting the rim from the hot gases and cooling it.

In addition, this inclination of the side wall of the rim makes iteasier to drill the cooling channels: it is easy to drill these channelseven in a vertical direction, since the space over the junction zonebetween the end wall and the side wall is sufficiently unencumbered toallow a laser beam or a tool (e.g. an electrode) to pass. Conversely, itcan be observed that drilling a channel at the base of a rim side wallforming an acute angle or a right angle with the end wall is anoperation that is lengthy and difficult since care needs to be taken toavoid damaging the side wall and to avoid piercing the channel too faraway from said wall.

In a particular embodiment of the invention, the blade does not have apressure rim extending the pressure wall, or it includes only a fractionof a pressure rim. Under such circumstances, the end of the bladepresents an end rim that extends between the leading edge and thetrailing edge of the blade but that is set back from the pressure side:in general this is constituted by the suction rim running along thesuction side, but it could also involve an intermediate rim situatedbetween the pressure and suction sides at the end of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages can be better understood on reading thefollowing detailed description of an embodiment of a blade in accordancewith the invention. This description is made with reference to theaccompanying figures, in which:

FIG. 1 is a perspective view of the free end of a blade presenting apressure rim and a suction rim;

FIG. 2 is a cross-section on plane II-II through the end of the FIG. 1blade, in a region that does not include a cooling channel;

FIG. 3 is a cross-section analogous to that of FIG. 2, through the freeend of a blade of the invention, which blade presents a pressure rim butnot a suction rim; this section is made through a cooling channel; and

FIG. 4 is a perspective view seen looking along arrow IV in FIG. 3showing the wall of the suction rim of the blade.

MORE DETAILED DESCRIPTION

With reference to FIGS. 3 and 4, there follows a description of anembodiment of a blade 102 of the invention. Portions of the blade 102that are analogous to portions of the blade 2 shown in FIGS. 1 and 2 aregiven the same reference numbers plus 100.

In the example of FIGS. 3 and 4, the free end 103 of the blade 102presents a suction rim 110 in the region that is shown, but it does notpresent a pressure rim.

The side wall 120 of the suction rim co-operates with the end wall 107to define the open cavity 105. Because there is no pressure rim, theside wall 120 is struck by the hot gas flowing through the turbine anddriving the blades 102. Relative to the blade, the hot gas flows in thedirection of arrow F. The side wall 120 is thus subjected to very hightemperatures and needs to be cooled effectively.

For this purpose, cooling channels 122 connect the internal coolingpassage 118 of the blade 102 to the cavity 105 and open out to the baseof the rim 110 at the junction zone between said rim and the end wall107 of the cavity 105. In this example, the side wall 120 and the endwall 107 present plane cross-sections, so there exists a sharp edge 130in the junction zone between these walls. Nevertheless, this junctionzone could be rounded.

The channels 122 comprise two portions: an adjustment portion formed bya drilled hole 124, and by a diffusion portion 126 formed both in linewith the hole 124 and by an indentation 128 formed in the wall 120 ofthe rim at the outlet from the channel 122. The adjustment portion is socalled since the minimum section of the drilled hole 124 influences thequantity of cool air flowing through the channel 122.

The diffusion portion 126 opens to the inside of the cavity 105 andcommunicates with the hole 124. The hole 124 opens out firstly to theinside of the diffusion portion 126 and secondly to the inside of thepassage 118.

The hole 124 may be cylindrical in shape and is made, for example, bylaser drilling or by electroerosion, at the base of the wall 120. Theaxis of the hole 124 is drawn in chain-dotted lines and is orthogonal tothe end wall 107.

The side wall 120 of the rim 110 forms an angle A relative to the endwall 107 that is obtuse, being strictly greater than 90°, so it does notimpede making the vertical hole 124.

The indentation 128 is formed in part in the wall 120 and in part insidethe hole 124. This indentation 128 is formed, for example, byelectroerosion using an electrode that is centered on the hole 124. Theelectrode may be conical in shape with a tip that is rounded to agreater or lesser extent. Thus, advantageously, the indentation 128presents an outline that is generally triangular in shape, and thebottom of this indentation is curved, and more precisely convex relativeto the drilling axis 124.

Its triangular outline serves to spread the flow of air leaving the hole124 and thus enlarge the area of the side wall 120 that is cooled.

The curved shape of the bottom of the indentation 128 serves to avoidforming sharp edges in the rim 110 since that would weaken it. Inaddition, the indentation 128 comes to an end before the top end of therim 110 so as to avoid weakening it. Advantageously, the indentation 128comes to an end before or approximately halfway up the rim 110 so thatthe cool air streams leaving the fan-shaped indentations 128 spread aswidely as possible over the wall 120.

Naturally, the way in which the cooling channels 122 are distributedalong the wall 120 is important for ensuring that this wall is properlycooled, and the channels 122 are preferably distributed uniformly andare close enough to one another to form a protective film that issubstantially continuous along the wall 120. The angle of inclination Aof the wall 120 is also important for ensuring that the cool air streamleaving the channels 122 remains in contact with the wall 120. Thus, theangle A should be small enough to ensure that the cool air streamflowing along arrow C does not move away from the wall 120, but on thecontrary remains in contact therewith.

Furthermore, the angle A should be large enough to avoid impedingdrilling the channels 122, as explained above. Thus, the angle A formedbetween the side wall 120 of the rim 110 and the end wall 107 lies inthe range 110° to 135°, and advantageously is situated at about 120°.

The above-described example blade is easy to manufacture, firstlybecause it is easy to make the sloping side wall 120 of the rim 110 bycasting, such that no additional machining step is needed. Secondly thecooling channels are made easier to drill by the wall 120 being inclinedand they can be made quickly using commonplace drilling techniques. Themanufacture of such a blade is thus inexpensive.

Naturally, the invention is not limited to a blade presenting a suctionrim only, but could be applied, for example, to a blade presenting asuction rim and a fraction only of a pressure rim, or indeed a bladepresenting an intermediate rim that does not extend either the pressurewall or the suction wall, but that is situated at the end of the blade,between said pressure and suction sides. Whatever the rim in question,the invention makes it possible to cool it by forming a film of cool airagainst its side wall that is exposed to the hot gas.

1. A hollow turbomachine blade including an internal cooling passage, anopen cavity situated at a free end of the blade and defined by an endwall and a side wall of at least one rim extending between a leadingedge and a trailing edge of the blade, and at least one cooling channelconnecting said internal cooling passage to said open cavity, whereinsaid cooling channel opens out at the base of said side wall of the rim,wherein said side wall forms an angle A relative to said end wall thatis obtuse, being strictly greater than 90°, wherein an indentation isformed in said side wall at an outlet from said cooling channel, andwherein said indentation comes to an end before the top end of the rim.2. A hollow blade according to claim 1, wherein the blade of theinvention does not have a pressure rim or has only a fraction of apressure rim.
 3. A hollow blade according to claim 1, wherein the angleformed between the side wall and the end wall lies in the range 110° to135°, and is advantageously situated at about 120°.
 4. A hollow bladeaccording to claim 1, wherein the outline of said indentation isgenerally triangular in shape.
 5. A hollow blade according to claim 1,wherein the bottom of said indentation is curved.
 6. A hollow bladeaccording to claim 1, wherein said cooling channel is in two portions:an adjustment portion formed by a drilled hole and a diffusion portionformed in part by said indentation.
 7. A hollow blade according to claim1, wherein a plurality of cooling channels are distributed regularlyalong said side wall.
 8. A hollow blade according to claim 1, whereinsaid indentation comes to an end close to half way up the height of saidrim.